1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which is in accordance with multi-echo imaging scheme and multi-slice imaging scheme.
2. Description of the Related Art
A multi-echo imaging scheme was developed recently in which multiple 180.degree. pulses are applied after application of a 90.degree. pulse to generate multiple spin-echo signals (hereinafter simply referred to as echo signals) per one excitation, and a different phase encoding amount is assigned to each of the echo signals, i.e., multiple echo signals are allocated to regions having different spatial frequencies, thereby acquiring a magnetic resonance (MR) image at high speed. One example of the multi-echo imaging scheme is known as "RARE (Rapid Acquisition with Relaxation Enhancement" imaging.
It has also been considered to combine the multi-echo imaging scheme with a multi-slice imaging scheme which acquires image data from multiple slices in one repetition time TR, that is, excites another or other slices in the interval that elapses from the acquisition of the last echo signal from one slice to the time when the next 90.degree. pulse for the one slice is applied (a 90.degree. pulse and multiple 180.degree. pulses for other slices are similarly applied). An example of the combined use of the multi-echo scheme and the multi-slice scheme has been disclosed and described in the specification of U.S. Pat. No. 4,818,940.
FIG. 1 shows a conventional pulse sequence in which the multi-echo and multi-slice imaging schemes are combined. The pulse sequence shown in FIG. 1 is intended to reconstruct an image of one slice by applying five 180.degree. pulses to the one slice after application of a 90.degree. pulse thereto to thereby generate five echo signals and acquiring the echo signals while varying their respective phase encoding amounts. That is, the first through fifth echo signals are allocated to five regions which differ in phase encoding amount on the spatial frequency plane. Two-dimensional Fourier transformation (2DFT) of the echo signals represented by spatial frequencies permits the reconstruction of an MR image of a slice.
The signal-to-noise ratio in an image is in inverse proportion to the square root of the bandwidth of echo signals. To improve the signal-to-noise ratio in image while preserving the same resolution, it is a common practice to narrow the bandwidth of echo signals by making the signal acquisition time ta1 longer and to make the magnitude of the readout gradient magnetic field Gr lower. In FIG. 2, there is shown a pulse sequence in which the bandwidth of echo signals is narrowed so as to improve the signal-to-noise ratio. In FIGS. 1 and 2, the axes of abscissa (time axis) are of the same scale. As can be seen from FIG. 2, therefore, the acquisition time ta2 for an echo signal is longer than the time ta1 in FIG. 1. The purpose of decreasing the magnitude of the readout gradient field Gr is to narrow the bandwidth (frequency) of one pixel while keeping the effective field of view. The magnitude of the readout gradient field Gr is determined in due consideration of the imhomogeneity in the static magnetic field and the generation of chemical artifacts.
By lowering the magnitude of the readout gradient field Gr to increase the echo-signal acquisition time from ta1 to ta2 in this manner, the signal-to-noise ratio can be increased by a factor of the square root of a2/a1.
In either of FIGS. 1 and 2, the time interval between 180.degree. pulses is constant (it is twice the time interval between the 90.degree. pulse and the first 180.degree. pulse). In FIG. 2, this time interval is 2.tau.1, which is longer than in FIG. 1.
As can be seen from FIGS. 1 and 2, however, increasing the echo-signal acquisition time results in increasing the time taken to acquire image data from one slice from ts1 to ts3. This will decrease the number of slices that can be obtained within a repetition time TR from TR/ts1 to TR/ts2. If the repetition time TR is made longer to keep the number of slices that can be obtained, the time taken to acquire image data is undesirably made longer. Thus, a magnetic resonance imaging apparatus which combines the multi-echo and multi-slice imaging schemes has a drawback that, if each echo acquisition time is made longer so as to improve the signal-to-noise ratio, the number of slices that can be acquired within a preselected repetition time TR is decreased. In other words, such a magnetic resonance imaging apparatus can improve the signal-to-noise ratio at the expense of the number of multiple slices acquired.